In recent years, the following semiconductor devices have been proposed: a horizontal element in which a current flows in a direction parallel to one main surface of a semiconductor substrate on the one main surface side; and a vertical element in which a current flows in a thickness direction of the semiconductor substrate. In the vertical element, it is effective to reduce the thickness of the semiconductor substrate in the range in which a desired breakdown voltage can be maintained, in order to reduce on-resistance during an operation to reduce electrical connection loss. In general, an element structure is formed on the front surface side of a thick semiconductor substrate and the rear surface of the semiconductor substrate is ground and polished to manufacture (produce) a thin semiconductor substrate (semiconductor chip). However, when the thickness of the semiconductor substrate is reduced, the mechanical strength of the semiconductor substrate is insufficient and there is a concern that the semiconductor substrate will be broken while a semiconductor chip is being manufactured.
The following Patent Document 1 discloses a semiconductor device including a metal-oxide-semiconductor field effect transistor (MOSFET) and a bipolar transistor. In the semiconductor device, a concave portion is provided in a portion of the rear surface of the chip in a MOSFET region and a drain electrode is provided on the bottom of the concave portion. According to this structure, it is possible to locally reduce the thickness of the semiconductor chip and to reduce on-resistance, while maintaining the mechanical strength of the entire semiconductor chip.
In the above-mentioned Patent Document 1, silicon (Si) is used as a semiconductor material. However, in recent years, silicon carbide (SiC) or gallium nitride (GaN) has been used. The reason is that SiC or GaN has a wider band gap than Si and the critical electric field intensity of SiC or GaN is ten times more than that of Si. Therefore, the thickness of a SiC chip or a GaN chip can be reduced to about a tenth of the thickness of a Si chip.
For example, in the case of an insulated gate bipolar transistor (IGBT) with a design breakdown voltage of 600 V to 1200 V, the Si chip requires a thickness of 70 μm to 180 μm and the SiC chip requires a small thickness of 20 μm or less. However, since the thin semiconductor chip has weak mechanical strength, it is difficult to treat the thin semiconductor chip without any change.
In order to solve the problems, the following Patent Document 2 to Patent Document 4 disclose a technique in which a concave portion is provided in the rear surface of a SiC chip to locally reduce the thickness of the SiC chip, thereby reducing on-resistance. For example, the following Patent Document 2 discloses a vertical MOSFET using SiC in which a concave portion is provided in the rear surface of the chip and a drain electrode is provided on the bottom of the concave portion to reduce on-resistance, similarly to the following Patent Document 1.
However, in the following Patent Document 2, the thickness of the SiC chip in a portion other than the portion in which the concave portion is provided is 400 μm and the thickness of the SiC chip in the portion in which the concave portion is provided is 200 μm. Therefore, features unique to SiC are not exhibited. The reason is that, when the depth of the concave portion increases and the thickness of the SiC chip in the portion in which the concave portion is provided is reduced in order to reduce the on-resistance, the mechanical strength of the SiC chip is reduced and it is difficult to change a process.
The following Patent Document 3 discloses a semiconductor device which includes a front surface element structure that is provided on the front surface side of a SiC chip, a plurality of concave portions that are provided in the rear surface of the chip opposite to the front surface element structure, and a net-shaped support (hereinafter, referred to as a rib) that surrounds the bottoms of the concave portions and forms the side walls of the concave portions. In this example, the rib has a net shape in a plan view. Therefore, it is possible to reduce on-resistance using the plurality of concave portions which are provided in the rear surface of the chip opposite to one front surface element structure, while preventing the breaking of the SiC chip during a process.
In the following Patent Document 4, similarly to the following Patent Document 2 and Patent Document 3, a concave portion is provided in the rear surface opposite to a front surface element structure which is provided on the front surface side of a SiC chip. According to this structure, it is possible to maintain the mechanical strength of the SiC chip and to reduce on-resistance. In this example, a rear surface metal electrode which is provided in the concave portion formed in the rear surface of the chip contributes to maintaining the mechanical strength of the SiC chip. The rear surface structure of the semiconductor chip according to the related art will be described with reference to FIG. 13. FIG. 13 is a diagram illustrating the rear surface structure of the semiconductor device according to the related art. FIG. 13(A) is a perspective view illustrating the rear surface structure of a semiconductor chip 100 as viewed from the rear surface, FIG. 13(B) is a cross-sectional view taken along the line C-C′ of FIG. 13(A), and FIG. 13(C) is a cross-sectional view taken along the line D-D′ of FIG. 13(A).
In FIG. 13, a rib 101 which has a predetermined width from the outer circumference of the semiconductor chip 100 is provided in an outer circumferential portion of the rear surface of the semiconductor chip 100. Here, the thickness x of the rib 101 may be equal to or greater than 30 μm, preferably, equal to or greater than 50 μm, in order to provide the rib 101 and to maintain the mechanical strength of the semiconductor chip 100. The thickness y of a central portion of the chip in which a front surface element structure 102 is provided may be equal to or less than 20 μm, in terms of a design breakdown voltage. Therefore, in a concave portion 103 which is provided in the rear surface of the semiconductor chip 100, a level difference z between the outer circumferential portion of the chip in which the rib 101 is provided and the central portion of the chip in which the front surface element structure 102 is provided is equal to or greater than 30 μm.
In general, the thickness of the rear surface metal electrode is about a few micrometers and a sufficient electrical effect is obtained. Therefore, the rear surface metal electrode is formed along the inner wall of the concave portion and has a shape which traces the concave portion.